fix(meade): build declination from wire components without losing the sign

[rbhbokka]

Follow-up to #300, which restored the DEC hemisphere conversion. The conversion itself is correct; the component assembly that feeds it is not.

The defect

Declination::fromCelestialDegrees() (src/Declination.cpp) builds the wire-seconds total as

const long wireSecs = ((60L * deg) + min) * 60L + sec;

meade::DecCoordinate is int16_t degrees; uint8_t minutes; uint8_t seconds; — minutes and seconds are unsigned magnitudes, and only degrees carries the sign. For a negative declination they are therefore added toward zero rather than away from it.

Error = 2 × (minutes × 60 + seconds) arcseconds, i.e. 0 to 1.9994°, on every celestial declination in (−90°, 0°) with non-zero arcminutes, in both hemispheres.

The setter and the getter disagree as a result, since getCelestialDegrees() splits correctly via core::DayTime::splitSeconds() while the join was hand-rolled — :Sd-05*30:00# followed by :Gd# returns -04*30:00.

wire	axis seconds	correct	error
-05*30:00	340200	343800	−1.000°
-24*23:00 (M8)	409020	411780	−0.767°
-69*06:00 (LMC)	572040	572760	−0.200°
-89*59:59	640801	647999	−1.999°

:CM sync goes through the same decFromWire() path, so a plate-solve sync at a negative declination writes the error into the mount's home reference rather than into a single slew.

These figures are derived by exercising the shipped fromCelestialDegrees() text against the real core:: conversions, not from a hardware session — I have not reproduced it on a mount.

Why the tests did not catch it

platformio.ini sets build_src_filter = +<./core> +<./ports> +<./adapters>, so src/Declination.cpp is never compiled into the native test binary. #300's tests exercise core::Declination::axisToCelestialSeconds, celestialToAxisSeconds and core::DayTime::splitSeconds — all three are correct and are unchanged here. The one piece of new arithmetic that landed outside src/core is the one that was wrong.

The fix

The asymmetry is the bug: getCelestialDegrees() delegates the split to the tested primitive core::DayTime::splitSeconds(), while fromCelestialDegrees() hand-rolled the join, because that inverse was never written.

This adds core::DayTime::joinSeconds() next to splitSeconds() as its exact inverse, refactors the existing DayTime(int, int, int) constructor onto it — that constructor already applied the sign correctly, so this lifts existing behaviour rather than introducing new arithmetic — and reduces fromCelestialDegrees() to pure composition, leaving no bare arithmetic outside core.

Refactoring the constructor onto the new primitive is deliberate: two pre-existing constructor tests now also guard the join, so the two can no longer drift apart.

Verification

• pio test -e native: 275 pass (269 on develop + 6 added).
• An exhaustive sweep of the domain readDecCoordinate accepts, both hemispheres, is now exact everywhere; before this change 1,771,746 of 3,543,852 inputs were wrong, worst case 14,400 arcsec.
• Builds clean under -Werror for oaeboardv1 (ESP32) and ramps (AVR, 16-bit int).

Not fixed here

DecCoordinate carries the sign only in degrees, so -00*30:00 is unrepresentable and still parses as +00*30:00 regardless of this change. That is a separate, wider issue across all three coordinate structs; it is noted in a comment here and addressed on fix/meade-sign-of-zero.

File overlap

None with the other Meade fixes I have open — this touches src/Declination.cpp, src/core/types/DayTime.* and the test_core/types tests only.